Display devices are used in portable computer systems, imaging systems, and other electronic devices. Many of these display devices require a source of light to illuminate a display screen. One example of a non-emissive display device that requires an external source of light is a liquid crystal display (“LCD”). LCDs typically include a liquid crystal layer containing liquid crystals which operate as light valves, allowing transmission of light in one state and blocking transmission of light in a second state. Placing a source of light or backlight behind the liquid crystal display and electronically controlling the switching of the light valves allows a user to view the front of the LCD to read text or images formed by the switched light valves. LCDs have become very popular in portable computing applications because they are rugged and require little space to operate. Moreover, LCDs are becoming increasingly popular for television applications.
Since last decade, due to technological breakthroughs, LEDs have become more and more important in lighting applications. A new dimmable backlight concept has been developed in which LEDs are driven by fast modulation, as a result of which the LEDs generate only the amount of light required for illuminating the image content. In this dimmable backlight concept, LEDs can selectively be switched on and off. The dimmable backlight overcomes the LCD's drawback of leakage light in dark areas of images and improves the contrast of the LCD. Backlight modulation can be achieved on the total backlight (0D-dimming), along horizontal segments (1D-dimming) or along horizontal and vertical segments (2D-dimming) using a matrix of individually addressable LEDs. The application of a conventional light guide design makes the boundary too sharp and not suitable to act as a segment or tile in a dimmable backlight application. At present, a straightforward manner to provide a dimmable illumination system driven by LEDs is to apply an illumination system comprising multiple LEDs positioned at a distance from an optical foil, wherein the LEDs are adapted to directly light the optical foil. A control unit is applied to selectively switch on and off the respective LEDs to enable selective dimming of the illumination system. To secure a substantially homogenous illumination of the optical foil, the distance, formed by an air gap, between the LEDs and the optical foil must be sufficiently large. Commonly the minimum thickness of the air gap is approximately equal to the pitch of the LEDs, wherein the pitch is defined as the distance between neighbouring LEDs. Since the minimum thickness is commonly between 2 and 5 cm, the known illumination system is relatively voluminous. There is, however, a permanent need to (further) reduce the thickness of the known illumination system, which may be used as a dimmable backlight unit.
It is an object of the invention to provide a relatively compact LED driven illumination system which is adapted to emit a relatively uniform light profile and which may be applied as a dimmable illumination system.